1. Field of the Invention
The present invention relates generally to resistors formed within and upon semiconductor integrated circuit substrates. More particularly, the present invention relates to a method whereby a polysilicon resistor formed within a semiconductor integrated circuit may be shielded from intrusion of hydrogen from sources internal or external to the integrated circuit.
2. Description of Related Art
In addition to the use of transistors and diodes as switching elements and current rectifying devices in semiconductor integrated circuits, it is also quite common in the art that those circuits will have resistors incorporated into their designs and fabrications. Commonly, a resistor within an integrated circuit will provide an electrical load which assures proper operation of the circuit under the electrical current and voltage parameters to which the circuit was designed.
There are several methods and materials through which resistors may be designed and fabricated into semiconductor integrated circuits. A common traditional method is to fabricate resistors within the silicon semiconductor substrate of an integrated circuit. Resistors of varying resistance can easily be fabricated within such semiconductor substrates by means of doping the semiconductor substrate through an ion implantation process similar to the ion implantation process used in forming other active semiconductor regions within the same semiconductor substrate. The practice of forming resistors through implanting dopant ions into semiconductor substrates is a viable technique unless resistors are required whose resistance exceeds the resistance of the semiconductor substrate within which the resistor is desired to be fabricated.
An alternate process which provides resistors of greater or equal resistance to resistors formed within semiconductor substrates is to form resistors from discrete portions of a highly resistive material formed upon the surface of a semiconductor substrate. A common high resistivity material which is useful in forming these higher resistance resistors is undoped or lightly doped polysilicon. While it is known in the art that doped polysilicon is an excellent conductor from which conductive structures within integrated circuit devices may be fabricated, it is also well known that undoped or lightly doped polysilicon is a highly resistive material.
An unfortunate characteristic of polysilicon resistors formed within integrated circuits is their ability to absorb hydrogen to which those resistors are exposed during semiconductor processing operations. This characteristic is well documented in the art. For example, Nakabayashi et al., in "Influence of Hydrogen on Electrical Characteristics of Poly-Si Resistor," 32 Jpn. J. Appl. Phys., Pt. 1, No. 9A, 3734 (1993), discusses the extent to which hydrogen may be absorbed into polysilicon resistors during hydrogen annealing operations to which those resistors are exposed. The disclosure also discusses the mechanism by which absorbed hydrogen affects the resistance of a polysilicon resistor.
In general, the disclosure indicates that hydrogen to which polysilicon resistors are exposed during integrated circuit processing is absorbed at trap locations within polysilicon grain boundaries of those resistors. Upon stabilization of the trap locations through absorption of hydrogen, the resistance of a polysilicon resistor may change substantially. In particular, Nakabayashi discloses that polysilicon resistors formed within semiconductor integrated circuits which integrated circuits have silicon nitride passivating layers upon their surfaces are particularly susceptible to substantial changes in resistivity upon subsequent annealing of the integrated circuits in a hydrogen atmosphere. Disclosed are decreases in polysilicon resistor resistance of up to 30%.
Also disclosed are methods by which some portion of the resistance decreases within those polysilicon resistors may be recovered. In particular, fabrication of a metal plate which covers a substantial portion of the polysilicon resistor surface and is immediately separated from the polysilicon resistor surface by a silicon oxide insulating layer may recover about one third of the 30% resistance decrease. An additional 10% of the resistance decrease may be recovered through forming the polysilicon resistor upon a thin oxide coating upon the semiconductor substrate upon which the polysilicon resistor is formed rather than forming the polysilicon resistor upon a thick field oxide coating upon the semiconductor substrate.
Not disclosed within the art are methods and materials which more completely stabilize polysilicon resistors against resistance losses due to hydrogen intrusion. Such methods and materials would be particularly useful for stabilizing polysilicon resistors which are formed within integrated circuits having silicon nitride passivation layers upon their surfaces when those integrated circuits are annealed in hydrogen atmospheres.